(1) Field of the Invention
This invention relates to an air/fuel ratio control system for an internal combustion engine, which controls the air/fuel ratio of the internal combustion engine by using, as feedback signals, detection signals from oxygen density sensors (hereinafter called "O.sub.2 sensors") arranged in the exhaust system of the internal combustion engine which may hereinafter be called "engine" as needed.
(2) Description of the Related Art
A variety of such air/fuel ratio control systems has heretofore been proposed for internal combustion engines. In air/fuel ratio control systems of the above sort for internal combustion engines, an O.sub.2 sensor which has been designed to change its output value abruptly near the stoichiometric fuel ratio by using the principle of oxygen concentration cells of a solid electrolyte, is arranged in an engine exhaust system at an upstream side relative to the point of arrangement of a catalytic converter (three-way catalyst) in the engine exhaust system. The air/fuel ratio of the internal combustion engine is controlled by comparing an output from the O.sub.2 sensor with a predetermined standard value (As the standard value, an intermediate value of values between which the abrupt change takes place is given as a fixed value. This value is useful as a value for the judgement of either a rich air-fuel mixture or a lean air-fuel mixture) and then controlling the quantity of the fuel to be injected from each electromagnetic fuel injection valve (injector) in such a way that the air-fuel mixture is rendered lean when the output of the O.sub.2 sensor is greater than the standard value but is rendered rich when the output of the O.sub.2 sensor becomes smaller on the contrary.
It has recently been proposed to provide an additional O.sub.2 sensor on the downstream side of the catalytic converter provided in the engine exhaust system (This O.sub.2 sensor will hereinafter be called "rearward O.sub.2 sensor" while an O.sub.2 sensor provided on the upstream side of the catalytic converter like the above-described O.sub.2 sensor will be called a forward O.sub.2 sensor) and to use an output from the rearward O.sub.2 as auxiliary information for the control of the air/fuel ratio (so-called dual O.sub.2 sensor system or double O.sub.2 sensor system). Even in this case, a standard value which should be compared with an output from the rearward O.sub.2 sensor will not be changed once it has been set.
Among such conventional air/fuel ratio control systems for internal combustion engines, as far as O.sub.2 sensors are concerned, the former systems perform the feedback control of the air/fuel ratio only by the output of the forward O.sub.2 sensor and there is hence a room for improvements to the accuracy of the control, and the latter systems may not be able to perform successfully the feedback control of the air/fuel ratio on the basis of the output of the forward O.sub.2 sensor in some instances because the standard value for the rearward O.sub.2 sensor is a fixed value, and there is also a room for improvements in this regard.
In the conventional air/fuel ratio control systems for internal combustion engines, the standard value to be compared with the output of the forward O.sub.2 sensor is a fixed value no matter whether they are of the former type or of the latter type. They hence involve a problem in connection with the reliability of the control, since the characteristics of O.sub.2 sensors vary from one sensor to another and also along the passage of time, the accuracy of the control varies, and the efficiency of cleaning of exhaust gas by the catalytic converter also changes.